Dual facing BSI image sensors with wafer level stacking

ABSTRACT

A device includes two BSI image sensor elements and a third element. The third element is bonded in between the two BSI image sensor elements using element level stacking methods. Each of the BSI image sensor elements includes a substrate and a metal stack disposed over a first side of the substrate. The substrate of the BSI image sensor element includes a photodiode region for accumulating an image charge in response to radiation incident upon a second side of the substrate. The third element also includes a substrate and a metal stack disposed over a first side of the substrate. The metal stacks of the two BSI image sensor elements and the third element are electrically coupled.

The present application is a divisional application of U.S. patentapplication Ser. No. 14/039,640, filed Sep. 27, 2013, which is herebyincorporated by reference in its entirety.

BACKGROUND

It is an ongoing trend that mobile electronic devices offer imagecapture capability. Some mobile electronic devices, such as a cellularphone, can capture images from both a front and a back side of thedevice. Many solutions exist for such dual facing camera capability.Solutions typically use two image sensors on opposing sides of thedevice.

Image sensors are integrated circuits (ICs) used to detect and measureradiation, such as light, received by the sensor device. A front-sideilluminated (FSI) image sensor typically has pixel circuitry and metalstacks disposed on a front side of a substrate where a photosensitive orphotodiode (“PD”) region resides. To form an image in the PD region, theradiation passes the metal stacks. A backside-illuminated (BSI) imagesensor, on the other hand, is typically formed on a thin substrate thatallows the radiation to reach the PD region by passing through thesubstrate. A BSI image sensor offers many advantages over an FSI imagesensor, such as shorter optical paths, higher quantum efficiency (QE),higher image resolution, smaller die sizes, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 illustrates an embodiment of an integrated camera module withdual-facing BSI image sensors.

FIG. 2 illustrates another embodiment of an integrated camera modulewith dual-facing BSI image sensors.

FIG. 3 is a flow chart for fabricating an integrated camera module withdual-facing BSI image sensors such as shown in FIG. 1, in accordancewith an embodiment.

FIGS. 4A-4H are cross sectional views of forming an integrated cameramodule with dual-facing BSI image sensors according to the method ofFIG. 3, in accordance with an embodiment.

FIG. 5 is a flow chart for fabricating an integrated camera module withdual-facing BSI image sensors such as shown in FIG. 2, in accordancewith an embodiment.

FIGS. 6A-6H are cross sectional views of forming an integrated cameramodule with dual-facing BSI image sensors according to the method ofFIG. 5, in accordance with an embodiment.

FIGS. 7A and 7B show an embodiment of an integration of a BSI imagesensor wafer with a processor wafer according to various aspects of thepresent disclosure.

FIG. 8 is a flow chart for fabricating an integrated camera module withdual-facing BSI image sensors, in accordance with an embodiment.

FIGS. 9A-9H are cross sectional views of forming an integrated cameramodule with dual-facing BSI image sensors according to the method ofFIG. 8, in accordance with an embodiment.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the disclosure.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. In addition, the present disclosuremay repeat reference numerals and/or letters in the various examples.This repetition is for the purpose of simplicity and clarity and doesnot in itself dictate a relationship between the various embodimentsand/or configurations discussed. Moreover, the performance of a firstprocess before a second process in the description that follows mayinclude embodiments in which the second process is performed immediatelyafter the first process, and may also include embodiments in whichadditional processes may be performed between the first and secondprocesses. Various features may be arbitrarily drawn in different scalesfor the sake of simplicity and clarity. Furthermore, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

Various embodiments of the present disclosure relate generally tointegration of two BSI image sensor wafers with a processor wafer toform a dual facing camera module using wafer level stacking methods.However, specific embodiments are provided as examples to teach thebroader inventive concept, and one of ordinary skill in the art caneasily apply the teaching of the present disclosure to other methods ordevice.

FIG. 1 illustrates an integrated camera module 100 according to variousaspects of the present disclosure. The camera module 100 includes aprocessor 120 bonded in between two BSI image sensors, 110 a and 110 b.The processor 120 includes an active layer 122 which includes activecircuit components such as transistors, and a metal stack 124 whichincludes interconnect structures for communicating within the processor120 as well as communicating between the processor 120 and the BSI imagesensors 110 a and 110 b. The BSI image sensors 110 a, 110 b are similarin that they each include a PD layer 112 a, 112 b on its back side and ametal stack 114 a, 114 b on its front side. In this embodiment, theprocessor 120 contacts the BSI image sensors 110 a and 110 b throughconductive features, 152, 154, 156 and 158, at their bonding surfaces.The camera module 100 also includes color filter and lens modules, 104 aand 104 b, disposed over the back side of the respective BSI imagesensors for collecting and filtering light. The camera module 100further includes cover glass elements, 102 a and 102 b, and dam elements106 a and 106 b, disposed over the color filter and lens modules 104 aand 104 b. The camera module 100 further includes bump elements 108 and109 for providing further integration of the camera module 100 to asubstrate (not shown). With such configuration as shown in FIG. 1, thecamera module 100 is able to capture radiation, such as light, incidentupon both sides (dual-facing).

FIG. 2 illustrates another integrated camera module 200 according tovarious aspects of the present disclosure. The camera module 200includes substantially the same elements as the camera module 100, witha processor 220 bonded in between two BSI image sensors, 210 a and 210b. In the camera module 200, bump elements 208 and 209 are electricallycoupled to the processor 220 using thru-silicon vias (TSVs), 230 and 232(FIG. 2), while the bump elements 108 and 109 of the camera module 100are electrically coupled to the processor 120 through the conductivefeatures 154 and 152 at the bonding surfaces of the processor 120 andthe BSI image sensor 110 b (FIG. 1).

FIG. 3 illustrates a process flow 300 for fabricating an integratedcamera module with dual-facing BSI image sensors, such as the cameramodule 100 (FIG. 1), according to various aspects of the presentdisclosure. FIG. 3 is best understood in conjunction with FIGS. 4A-4H.

The process flow 300 (FIG. 3) receives a processor wafer (operation 302)and a first BSI image sensor wafer (operation 304). Referring to FIG.4A, an exemplar processor wafer 420 includes a substrate 421 and a metalstack 424 formed over the substrate 421. The substrate 421 includes anactive region 422. The processor wafer 420 has two surfaces, 444 and446. The surface 444 is at a front side of the metal stack 424 and thesurface 446 is at a back side of the substrate 421. The surface 444includes conductive pads, 426 and 428, isolated by a dielectric materiallayer. The conductive pads, 426 and 428, may contain a metal, such ascopper. The dielectric material layer may include silicon oxide, siliconnitride, silicon oxynitride, a low-k material, or another suitabledielectric material. A thickness of the dielectric material layer isselected so that the dielectric material layer will effectively blockmigration of a metal applied to the surface 444 during a later bondingprocess. This will be described in more detail below. In an embodiment,the substrate 421 includes silicon. Alternatively, the substrate 421 mayinclude another suitable semiconductor material. The active region 422includes active and passive circuit components, such as field effecttransistors (FETs), complementary metal-oxide semiconductor (CMOS)transistors, FinFETs, high voltage transistors, high frequencytransistors, bipolar junction transistors, resistors, capacitors,diodes, fuses, other suitable devices, and/or combinations thereof. Themetal stack 424 includes multilayer interconnect structures forelectrically coupling circuit components in the active region 422. Theinterconnect structures include various conductive features, such ascontacts, vias, and/or conductive traces. The various conductivefeatures include materials such as copper, aluminum,aluminum/silicon/copper alloy, titanium, titanium nitride, tungsten,polysilicon, metal silicide, and/or combinations thereof. The processorwafer 420 is provided merely as an example, and its exact compositionand/or functionality do not limit the inventive principles of thepresent disclosure.

Referring again to FIG. 4A, a first BSI image sensor wafer 410 aincludes a substrate 411 a and a metal stack 414 a formed over thesubstrate 411 a. The substrate 411 a includes a photosensitive orphotodiode (“PD”) region 412 a. The BSI wafer 410 a has two surfaces,404 a and 406 a. The surface 404 a is at a front side of the metal stack414 a and the surface 406 a is at a back side of the substrate 411 a.The surface 404 a includes conductive pads, 416 a and 418 a, isolated bya dielectric material about the same as the dielectric material of thesurface 444. The conductive pads, 416 a and 418 a, use about the samematerial as that of the conductive pads 426 and 428. The metal stack 414a may include one or more layers of metal separated by inter-layerdielectric (ILD) layers.

The substrate 411 a has an initial thickness 413 a. In some embodiments,the initial thickness 413 a is in a range from about 100 microns (μm) toabout 3000 μm, for example between about 500 μm and about 1000 μm.Radiation, such as light, is projected from the back side and enters thesubstrate 411 a through the surface 406 a.

In some embodiments, the substrate 411 a includes an elementarysemiconductor such as silicon or germanium and/or a compoundsemiconductor, such as silicon germanium, silicon carbide, galliumarsenic, indium arsenide, gallium nitride, and indium phosphide. Otherexemplary substrate materials include alloy semiconductors, such assilicon germanium carbide, gallium arsenic phosphide, and gallium indiumphosphide. The substrate 411 a may also include non-semiconductormaterials including soda-lime glass, fused silica, fused quartz, calciumfluoride (CaF₂), and/or other suitable materials. In some embodiments,the substrate 411 a has one or more layers defined within it, such as anepitaxial layer. For example, in one such embodiment, the substrate 411a includes an epitaxial layer overlying a bulk semiconductor. Otherlayered substrates include semiconductor-on-insulator (SOI) substrates.In one such SOI substrate, the substrate 411 a includes a buried oxide(BOX) layer formed by a process such as separation by implanted oxygen(SIMOX). In various embodiments, the substrate 411 a may take the formof a planar substrate, a fin, a nanowire, and/or other forms.

The substrate 411 a may include one or more doped regions. For example,the substrate 411 a may be doped with a p-type dopant. Suitable p-typedopants include boron, gallium, indium, other suitable p-type dopants,and/or combinations thereof. Alternatively, the substrate 411 a mayinclude one or more regions doped with an n-type dopant such asphosphorus, arsenic, other suitable n-type dopants, and/or combinationsthereof. Doping may be implemented using a process such as ionimplantation or diffusion in various steps and techniques.

The PD region 412 a includes one or more sensor element which may bestandalone or an integral part of a larger pixel array, such as thearray commonly found in a digital camera sensor. The sensor elementdetects the intensity (brightness) of radiation incident upon the backsurface 406 a of the substrate 411 a. In some embodiments, the incidentradiation is visual light. Alternatively, the incident radiation may beinfrared (IR), ultraviolet (UV), x-ray, microwave, other suitableradiation, and/or combinations thereof. The sensor element may beconfigured to respond to particular wavelengths or ranges ofwavelengths, such as red, green, and blue wavelengths within the visiblelight spectrum. The sensor element(s) in the PD region 412 a may beformed in the substrate 411 a by a method such as diffusion and/or ionimplantation.

Although not shown, the substrate 411 a includes other structures ordevices, such as a pixel circuitry for controlling and communicatingwith the PD region 412 a for image acquisition, shallow trenchisolations (STIs) for isolating sensor elements, and other passive oractive devices.

The process flow 300 (FIG. 3) proceeds to operation 306 where theprocessor wafer 420 is aligned and bonded with the BSI wafer 410 a usinga hybrid bond process. Referring to FIG. 4B, the surface 444 is directlybonded with the surface 404 a with the conductive pads 428 and 426bonded with the conductive pads 418 a and 416 a respectively. A hybridbond process refers to bonding of two surfaces where each surfaceincludes at least two substantially different materials (a hybridsurface). In the present embodiment, the conductive pads, 426, 428, 416a, and 418 a, contain a metal, such as copper, while the surfaces 444and 404 a contain a layer of dielectric material such as oxide, siliconoxide, silicon nitride, or silicon oxynitride. In an embodiment, thesurfaces 444 and 404 a contain a layer of silicon oxynitride having athickness of at least 30 nanometers (nm) so as to effectively blockmigration of copper if the two surfaces are misaligned. In anotherembodiment, the surfaces 444 and 404 a contain a layer of siliconnitride having a thickness of at least 5 nm so as to effectively blockmigration of copper if the two surfaces are misaligned.

In the present embodiment, the hybrid bond process includes an initialbonding process at a lower temperature followed by an annealing processat an elevated temperature. The initial bonding process may use atechnique such as a direct bonding or other bonding techniques.Generally, it takes longer, e.g. a few hours, for the initial bonds toform. Therefore, a lower temperature is used during the initial bondingprocess to avoid undesirable changes or decomposition in the wafers 420and 410 a. In an embodiment, the initial bonding process may take placeat room temperature or another temperature that is below 200 degreesCelsius. In an embodiment, pressure is applied to the wafers 420 and 410a for the initial bonds to form. The annealing process is applied afterthe initial bonding process to strengthen the bonds between the twohybrid surfaces 404 a and 444. In the present embodiment, the annealingprocess undergoes at least two stages of bond formation at temperatureshigher than the initial bonding temperature. During a first stage,covalent bonds are formed between the dielectric materials, e.g. betweentwo oxides, of the surfaces 420 and 410 a at a first temperature, suchas about 200 degrees Celsius. In an embodiment, the first stage takesabout one and half hours. During a second stage, metal bonding areformed, e.g. by copper inter-diffusion, at a second temperature that ishigher than the first temperature, such as about 350 degrees Celsius. Inan embodiment, the second stage takes about half an hour. Thetemperatures of the annealing process are generally under about 450degrees Celsius, as higher temperature may lead to damages in the wafers420 and 410 a. However, the specific temperatures and durationsdisclosed above are mere examples and do not limit the inventive scopeof the present disclosure. Moreover, in the present embodiment, bondingof the conductive pads 426/416 a and 428/418 a is for illustrativepurposes only and does not indicate a specific orientation of the BSIwafer 410 a with respect to the processor wafer 420.

The process flow 300 (FIG. 3) proceeds to operation 308 where the firstBSI wafer 410 a is thinned down. Referring to FIG. 4C, a thinningprocess is applied to thin down the BSI wafer substrate 411 a from itsback side surface 406 a. The thinning process may include a mechanicalgrinding process and a chemical thinning process. A substantial amountof substrate material may be first removed from the substrate 411 aduring the mechanical grinding process. Afterwards, the chemicalthinning process may apply an etching chemical to the back side of thesubstrate 411 a to further thin the substrate 411 a to a thickness 415a, which may be on the order of a few microns (μm). The thickness 415 aaffects a quantum efficiency of the BSI image sensors in the wafer 410a. In some embodiments, the thickness 415 a is selected to improve thequantum efficiency of the BSI image sensors. In some embodiments, thethickness 415 a is greater than about 1 μm but less than about 5 μm. Theparticular thicknesses disclosed in the present disclosure are mereexamples and other thicknesses may be implemented depending on the typeof application and design of the integrated camera module 100 (FIG. 1).

The process flow 300 (FIG. 3) proceeds to operation 310 where thebackside of the substrate 421 undergoes a metallization process.Referring to FIG. 4D, a passivation layer 430 is formed over thesubstrate 421 using a suitable process such as a process including adeposition process and a chemical mechanical polishing (CMP) process. Inan embodiment, the passivation layer 430 includes a dielectric material,such as oxide or silicon oxide. Conductive features, 436, 438, 432 and434, are further formed into the passivation layer 430 and through thesubstrate 421 for coupling the metal stack 424 to the passivation layer430. The process of forming the conductive features includes etching thevarious layers to form thru-layer or thru-silicon vias and/or contacttrenches; depositing a conductive material, such as copper, into thevias and/or trenches; and performing a CMP process to the conductivematerial.

The process flow 300 (FIG. 3) proceeds to operation 312 where a secondBSI wafer 410 b is received. Referring to FIG. 4E, the second BSI wafer410 b includes a substrate 411 b and a metal stack 414 b formed over thesubstrate 411 b. The substrate 411 b has a thickness 413 b and includesa photosensitive or photodiode (“PD”) region 412 b. The BSI wafer 410 bhas two surfaces, 404 b and 406 b. The surface 404 b is at a front sideof the metal stack 414 b and the surface 406 b is at a back side of thesubstrate 411 b. The surface 404 b includes conductive pads, 416 b and418 b, isolated by a dielectric material about the same as thedielectric material of the surface 446. The conductive pads, 416 b and418 b, use about the same material as that of the conductive pads 436and 438. The metal stack 414 b may include one or more layers of metalseparated by inter-layer dielectric (ILD) layers. The BSI wafer 410 bmay use a composition similar to or different from the BSI wafer 410 a.Moreover, the BSI wafers 410 a and 410 b may contain the same ordifferent number of imaging pixels.

The process flow 300 (FIG. 3) proceeds to operation 314 where the secondBSI wafer 410 b is aligned and bonded with the processor wafer 420 usinga hybrid bond process. Referring to FIG. 4F, the surface 404 b of theBSI wafer 410 b is directly bonded with the surface 446 of the processorwafer 420 with the conductive pads 418 b and 416 b on the BSI wafer 410b bonded with the conductive pads 438 and 436 on the processor wafer 420respectively. The hybrid bond process in this operation may besubstantially similar to the hybrid bond process in operation 306, withtemperatures and duration suitable for the material/composition of thesurfaces 446 and 404 b. The operation 314 thus produces an assembly 400with the processor wafer 420 bonded in between the BSI wafers 410 a and410 b.

The process flow 300 (FIG. 3) proceeds to operation 316 where the secondBSI wafer 410 b is thinned down. Referring to FIG. 4G, a thinningprocess is applied to thin down the BSI wafer substrate 411 b from itsback side surface 406 b. The thinning process in this operation may besubstantially similar to the thinning process in operation 308. Thesubstrate 411 b is thinned to a thickness 415 b, which may be on theorder of a few microns (μm). In some embodiments, the thickness 415 b isgreater than about 1 μm but less than about 5 μm. The thickness 415 bmay be similar to or different from the thickness 415 a depending on thetype of application and design of the integrated camera module 100 (FIG.1).

The process flow 300 (FIG. 3) proceeds to operation 318 where conductivefeatures are formed on the back side of the BSI wafer 410 b (or 410 a)so that the assembly 400 may be further integrated with other componentsof the integrated camera module 100 (FIG. 1). Referring to FIG. 4H,conductive features 458 and 456 are formed into the substrate 411 b andare coupled to the metal stack 414 b and/or the metal stack 424. Theprocess of forming the conductive features 458 and 456 includes etchingthe substrate 411 b to form thru-layer or thru-silicon vias and/orcontact trenches; depositing a conductive material, such as copper, intothe vias and/or trenches; and performing a polishing process to theconductive material. Either the back side of the BSI wafer 410 b or theback side of the BSI wafer 410 a may be used for operation 318.

The process flow 300 (FIG. 3) proceeds to operation 320 to complete theintegrated camera module 100 (FIG. 1). Operation 320 may include formingcolor filters and lens over both sides of the assembly 400, installingglass covers over the color filters and lens, installing package ballsover the conductive pads 448 and 446 for further integration, and so on.

FIG. 5 illustrates a process flow 500 for fabricating an integratedcamera module with dual-facing BSI image sensors, such as the cameramodule 200 (FIG. 2), according to various aspects of the presentdisclosure. FIG. 5 is best understood in conjunction with FIGS. 6A-6H.For simplicity purposes, where an operation in the process flow 500 issimilar to an operation in the process flow 300, a reference to theprocess flow 300 is made and differences are highlighted.

The process flow 500 (FIG. 5) receives a processor wafer (operation 502)and a first BSI image sensor wafer (operation 504). Referring to FIG.6A, an exemplar processor wafer 620 includes a substrate 621 and a metalstack 624 formed over the substrate 621. The substrate 621 includes anactive region 622. The processor wafer 620 has two surfaces, 644 and646. The surface 644 is at a front side of the metal stack 624 and thesurface 646 is at a back side of the substrate 621. The surface 644includes conductive pads, 626 and 628, isolated by a dielectricmaterial. The structure and composition of the processor wafer 620 issimilar to the processor wafer 420 (FIG. 4A). Also shown in FIG. 6A isan exemplar BSI image sensor wafer 610 a. The BSI wafer 610 a includes asubstrate 611 a and a metal stack 614 a formed over the substrate 611 a.The substrate 611 a includes a photosensitive or photodiode (“PD”)region 612 a. The BSI wafer 610 a has two surfaces, 604 a and 606 a. Thesurface 604 a is at a front side of the metal stack 614 a and thesurface 606 a is at a back side of the substrate 611 a. The surface 604a includes conductive pads, 616 a and 618 a, isolated by a dielectricmaterial about the same as the dielectric material of the surface 644.The substrate 611 a has an initial thickness 613 a. The structure andcomposition of the BSI wafer 610 a is similar to the BSI wafer 410 a(FIG. 4A).

The process flow 500 (FIG. 5) proceeds to operation 506 where theprocessor wafer 620 is aligned and bonded with the BSI wafer 610 a usinga hybrid bond process. Referring to FIG. 6B, the surface 644 is directlybonded with the surface 604 a with the conductive pads 628 and 626bonded with the conductive pads 618 a and 616 a respectively. The hybridbond process in this operation is similar to the hybrid bond process inoperation 306 (FIG. 3).

The process flow 500 (FIG. 5) proceeds to operation 508 where the firstBSI wafer 610 a is thinned down. Referring to FIG. 6C, a thinningprocess similar to the thinning process in operation 308 (FIG. 3) isapplied to thin down the BSI wafer substrate 611 a from its back sidesurface 606 a to a thickness 615 a, which may be on the order of a fewmicrons (μm). In some embodiments, the thickness 615 a is greater thanabout 1 μm but less than about 5 μm. The particular thicknessesdisclosed in the present disclosure are mere examples and otherthicknesses may be implemented depending on the type of application anddesign of the integrated camera module 200 (FIG. 2).

The process flow 500 (FIG. 5) proceeds to operation 512 where a secondBSI wafer 610 b is received. Referring to FIG. 6D, the second BSI wafer610 b includes a substrate 611 b and a metal stack 614 b formed over thesubstrate 611 b. The substrate 611 b has a thickness 613 b and includesa PD region 612 b. The BSI wafer 610 b has two surfaces, 604 b and 606b. The surface 604 b is at a front side of the metal stack 614 b and thesurface 606 b is at a back side of the substrate 611 b. A differencebetween the BSI wafer 610 b and the second BSI wafer 410 b (FIG. 4E)received in the process flow 300 (FIG. 3) is that the surface 604 b doesnot include conductive features and only includes a material which isabout the same as the material of the surface 646 on the back side ofthe processor wafer substrate 621. The BSI wafers 610 a and 610 b maycontain the same or a different number of imaging pixels.

The process flow 500 (FIG. 5) proceeds to operation 514 where the secondBSI wafer 610 b is aligned and bonded with the processor wafer 620 usinga fusion bond process. Referring to FIG. 6E, the surface 646 is directlybonded with the surface 604 b. A fusion bond process refers to bondingof two surfaces where the two surfaces have about same material. In anembodiment, the two surfaces, 646 and 604 b, include silicon or siliconoxide. In another embodiment, the two surfaces, 646 and 604 b, includesilicon oxynitride or silicon nitride. Other materials or combinationssuitable for direct bonding may be used for the two surfaces 646 and 604b. The fusion bond process includes an initial bonding process followedby an annealing process. The initial bonding process may use a directbonding technique or other bonding techniques and is performed at asuitable temperature, such as below 200 degrees Celsius. The annealingprocess is used to strengthen the bonds between the two surfaces 646 and604 b. In an embodiment, the annealing process is used to converthydrogen bonds formed in the initial bonding process to covalent bondsat a suitable temperature, such as about 200 degrees Celsius. Theoperation 514 thus produces an assembly 600 with the processor wafer 620bonded in between the BSI wafers 610 a and 610 b.

The process flow 500 (FIG. 5) proceeds to operation 516 where the secondBSI wafer 610 b is thinned down. Referring to FIG. 6F, a thinningprocess is applied to thin down the BSI wafer substrate 611 b from itsback side surface 606 b. The thinning process in this operation issimilar to the thinning process in operation 316 (FIG. 3). The substrate611 b is thinned to a thickness 615 b, which may be on the order of afew microns (μm). In some embodiments, the thickness 615 b is greaterthan about 1 μm but less than about 5 μm. The thickness 615 b may besimilar to or different from the thickness 615 a depending on the typeof application and design requirements of the integrated camera module200 (FIG. 2).

The process flow 500 (FIG. 5) proceeds to operation 518 where conductivepads are formed over the surface 606 b and thru-layer and/orthru-silicon vias are formed to electrically couple the conductive padsto both the second BSI wafer 610 b and the processor wafer 620.Referring to FIG. 6G, in the present embodiment, operation 518 etchesthe back side of the substrate 611 b for defining openings forconductive pads, etches through the substrate 611 b for definingopenings for vias contacting the metal stack 614 b, and etches throughboth the second BSI wafer 610 b and the substrate 621 for definingopenings for vias contacting the metal stack 624. Referring to FIG. 6H,operation 518 proceeds to forming an isolation layer in the openings bya process, such as deposition; etching the isolation layer; depositing aconductive material into the etched isolation layer; and performing apolishing process, such as a CMP process, to the conductive material toform the conductive pads, 662 and 672, and the thru-silicon vias, 664,666, 674 and 676. Other embodiments of forming the conductive pads 662and 672 and electrically coupling them to the metal stacks 614 b and 624are possible.

The process flow 500 (FIG. 5) proceeds to operation 520 to complete theintegrated camera module 200 (FIG. 2). Operation 520 may include formingcolor filters and lens over both sides of the assembly 600; installingglass covers over the color filters and lens; installing package ballsover the conductive pads, 662 and 672 for further integration; and soon.

In both the process flows, 300 and 500, a hybrid bond process is used tobond a BSI wafer to a processor wafer, such as illustrated in FIGS. 4B,4F and 6B. FIGS. 7A and 7B illustrate a method of using a redistributionlayer during such a hybrid bond process.

Referring to FIG. 7A, a redistribution layer 752 is formed over a metalstack 724 of a processor wafer 720. The redistribution layer 752includes conductive pads 756 and 758 that are electrically coupled tothe conductive pads 726 and 728 and substantially extend surface areasof the conductive pads 726 and 728 respectively. The conductive pads 756and 758 are isolated by a dielectric material, such as oxide. Theprocess of forming the redistribution layer 752 includes depositing thedielectric material over the metal stack 724, etching the dielectricmaterial for defining openings for the conductive pads 756 and 758,filling the openings with a conductive material such as copper, andperforming a polishing process, such as a CMP process, to the conductivematerial. FIG. 7A also shows a BSI wafer 710 a with a bonding surface704 a and two conductive pads 716 a and 718 a on the bonding surface 704a.

Referring to FIG. 7B, the BSI wafer 710 a is aligned and bonded to theprocessor wafer 720 with the redistribution layer 752 providing anotherbonding surface. Since the conductive pad 756 (or 758) has asubstantially larger surface area than the conductive pad 716 a (or 718a), using the redistribution layer 752 generally provides benefits forincreasing design tolerance of the conductive pad 716 a (or 718 a) andincreasing design tolerance of the alignment operation during the hybridbond process. A redistribution layer, such as the layer 752, may be partof the processor wafer 720 when the processor wafer 720 is received,such as in the operation 302 (FIG. 3) and the operation 502 (FIG. 5).Alternatively, the processor wafer 720 may be processed to include theredistribution layer 752 after it is received and before it is bondedwith the BSI wafer. Alternatively, a redistribution layer may be formedover a metal stack of a BSI wafer before it is bonded to a metal stackof a processor wafer.

FIG. 8 illustrates a process flow 800 for fabricating an integratedcamera module with dual-facing BSI image sensors according to variousaspects of the present disclosure. The process flow 800 is similar tothe process flow 500 (FIG. 5), with differences discussed below. Onedifference is that both BSI sensors are bonded with a processor waferusing fusion bond processes in the process flow 800. FIG. 8 can bebetter understood when discussed with an example device, as shown inFIGS. 9A-9H. For simplicity purposes, where an operation in the processflow 800 is similar to an operation in the process flow 500, a referenceto the process flow 500 is made and differences are highlighted.

The process flow 800 receives a processor wafer (operation 802) and afirst BSI image sensor wafer (operation 804). Referring to FIG. 9A, anexemplar processor wafer 920 includes a substrate 921 and a metal stack924 formed over the substrate 921. The substrate 921 includes an activeregion 922. The processor wafer 920 has two surfaces, 944 and 946. Thesurface 944 is at a front side of the metal stack 924 and the surface946 is at a back side of the substrate 921. The surface 944 includes adielectric material. The structure and composition of the processorwafer 920 is similar to the processor wafer 620 (FIG. 6A). A differenceis that the surface 944 does not include conductive pads. Also shown inFIG. 9A is an exemplar BSI image sensor wafer 910 a. The BSI wafer 910 aincludes a substrate 911 a and a metal stack 914 a formed over thesubstrate 911 a. The substrate 911 a includes a photosensitive orphotodiode (“PD”) region 912 a. The BSI wafer 910 a has two surfaces,904 a and 906 a. The surface 904 a is at a front side of the metal stack914 a and the surface 906 a is at a back side of the substrate 911 a.The surface 904 a includes a dielectric material about the same as thedielectric material of the surface 944. The substrate 911 a has aninitial thickness 913 a. The structure and composition of the BSI wafer910 a is similar to the BSI wafer 610 a (FIG. 6A). A difference is thatthe surface 904 a does not include conductive pads.

The process flow 800 (FIG. 8) proceeds to operation 806 where theprocessor wafer 920 is aligned and bonded with the BSI wafer 910 a usinga fusion bond process. Referring to FIG. 9B, the surface 944 is directlybonded with the surface 904 a. The fusion bond process in this operationis similar to the fusion bond process in operation 514 (FIG. 5).

The process flow 800 (FIG. 8) proceeds to operation 808 where the firstBSI wafer 910 a is thinned down. Referring to FIG. 9C, a thinningprocess similar to the thinning process in operation 508 (FIG. 5) isapplied to thin down the BSI wafer substrate 911 a from its back sidesurface 906 a to a thickness 915 a, which may be on the order of a fewmicrons (μm). In some embodiments, the thickness 915 a is greater thanabout 1 μm but less than about 5 μm. The particular thicknessesdisclosed in the present disclosure are mere examples and otherthicknesses may be implemented depending on the type of application anddesign of the integrated camera module to be fabricated.

The process flow 800 (FIG. 8) proceeds to operation 812 where a secondBSI wafer 910 b is received. Referring to FIG. 9D, the second BSI wafer610 b includes a substrate 911 b and a metal stack 914 b formed over thesubstrate 911 b. The substrate 911 b has a thickness 913 b and includesa PD region 912 b. The BSI wafer 910 b has two surfaces, 904 b and 906b. The surface 904 b is at a front side of the metal stack 914 b and thesurface 906 b is at a back side of the substrate 911 b. The surface 904b includes a material which is about the same as the material of thesurface 946. The BSI wafers 910 a and 910 b may contain the same or adifferent number of imaging pixels.

The process flow 800 (FIG. 8) proceeds to operation 814 where the secondBSI wafer 910 b is aligned and bonded with the processor wafer 920 usinga fusion bond process. Referring to FIG. 9E, the surface 946 is directlybonded with the surface 904 b. The fusion bond process in this operationis similar to the fusion bond process in operation 514 (FIG. 5).

The process flow 800 (FIG. 8) proceeds to operation 816 where the secondBSI wafer 910 b is thinned down. Referring to FIG. 9F, a thinningprocess is applied to thin down the BSI wafer substrate 911 b from itsback side surface 906 b. The thinning process in this operation issimilar to the thinning process in operation 516 (FIG. 5). The substrate911 b is thinned to a thickness 915 b, which may be on the order of afew microns (μm). In some embodiments, the thickness 915 b is greaterthan about 1 μm but less than about 5 μm. The thickness 915 b may besimilar to or different from the thickness 915 a depending on the typeof application and design requirements of the integrated camera moduleto be fabricated.

The process flow 800 (FIG. 8) proceeds to operation 818 where conductivepads and thru-layer and/or thru-silicon vias are formed to electricallycouple both the BSI wafers 910 a and 910 b to the processor wafer 920.FIG. 9G illustrates that the wafers 910 a, 910 b and 920 are etched.FIG. 9H illustrates that conductive features 962, 964, 966, 972, 974 and976 are formed to couple the metal stacks in the three wafers. Theprocesses of etching the wafers and forming the conductive features aresimilar to those in operation 518 (FIG. 5).

The process flow 800 (FIG. 8) proceeds to operation 820 to complete theintegrated camera module. Operation 820 may include forming colorfilters and lens over both sides of the assembly 900; installing glasscovers over the color filters and lens; and so on.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

In one exemplary aspect, the present disclosure is directed to a deviceincluding a first BSI image sensor, a second BSI image sensor, and athird element. The first BSI image sensor includes a first substrate anda first metal stack disposed over a first side of the first substrate.The first substrate includes a photodiode region for accumulating animage charge in response to radiation incident upon a second side of thefirst substrate. The first metal stack is operatively coupled to thefirst substrate for receiving image data from the first substrate. Thefirst metal stack includes a first material layer at a first side of thefirst metal stack. The second BSI image sensor includes a secondsubstrate and a second metal stack disposed over a first side of thesecond substrate. The second substrate includes a photodiode region foraccumulating an image charge in response to radiation incident upon asecond side of the second substrate. The second metal stack isoperatively coupled to the second substrate for receiving image datafrom the second substrate. The second metal stack includes a secondmaterial layer at a first side of the second metal stack. The thirdelement includes a third substrate and a third metal stack disposed overa first side of the third substrate. The third substrate includes anactive region. The third metal stack includes a third material layer ata first side of the third metal stack. The first side of the first metalstack is bonded to the first side of the third metal stack and the firstmetal stack is electrically coupled to the third metal stack. The firstside of the second metal stack is bonded to a second side of the thirdsubstrate and the second metal stack is electrically coupled to thethird metal stack.

In another exemplary aspect, the present disclosure is directed to amethod for fabricating a dual facing BSI image sensor assembly. Themethod includes receiving a first BSI image sensor element, a second BSIimage sensor element, and a third element. The first BSI image sensorelement includes a first substrate and a first metal stack formed over afirst side of the first substrate. The first substrate includes aphotodiode region for sensing radiation incident upon a second side ofthe first substrate. A first side of the first metal stack includes afirst plurality of conductive features. The second BSI image sensorelement includes a second substrate and a second metal stack formed overa first side of the second substrate. The second substrate includes aphotodiode region for sensing radiation incident upon a second side ofthe second substrate. A first side of the second metal stack includes asecond plurality of conductive features. The third element includes athird substrate and a third metal stack formed over a first side of thethird substrate. A first side of the third metal stack includes a thirdplurality of conductive features. The method further includes bondingthe first side of the first metal stack to the first side of the thirdmetal stack using a first hybrid bond process and thinning the firstsubstrate from the second side of the first substrate to a firstthickness. The method further includes forming a passivation layer overa second side of the third substrate, wherein a first side of thepassivation layer includes a fourth plurality of conductive featuresthat is electrically coupled to the third metal stack. The methodfurther includes bonding the first side of the second metal stack to thefirst side of the passivation layer using a second hybrid bond processand thinning the second substrate from the second side of the secondsubstrate to a second thickness.

In another exemplary aspect, the present disclosure is directed to amethod for fabricating an integrated camera module having dual facingBSI image sensors. The method includes receiving a first BSI imagesensor element, a second BSI image sensor element, and a third element.The first BSI image sensor element includes a first substrate and afirst metal stack formed over a first side of the first substrate. Thefirst substrate includes a photodiode region for sensing radiationincident upon a second side of the first substrate. The second BSI imagesensor element includes a second substrate and a second metal stackformed over a first side of the second substrate. The second substrateincludes a photodiode region for sensing radiation incident upon asecond side of the second substrate. The third element includes a thirdsubstrate and a third metal stack formed over a first side of the thirdsubstrate. The method further includes bonding a first side of the firstmetal stack to a first side of the third metal stack and thinning thefirst substrate from the second side of the first substrate to a firstthickness. The method further includes bonding a first side of thesecond metal stack to a second side of the third substrate layer using afirst fusion bond process; thinning the second substrate from the secondside of the second substrate to a second thickness; and formingconductive features over the second side of the second substrate,wherein the conductive features electrically couple the second metalstack to the third metal stack.

What is claimed is:
 1. A device comprising: a first image sensor elementhaving a first substrate and a first metal stack disposed over a firstside of the first substrate, the first substrate including a firstphotodiode region; a second image sensor element having a secondsubstrate and a second metal stack disposed over a first side of thesecond substrate, the second substrate including a second photodioderegion; a third element disposed between the first and second imagesensors and having a third substrate and a third metal stack disposedover a first side of the third substrate, wherein a first side of thefirst metal stack is bonded to a first side of the third metal stack anda second side of the third substrate is bonded to a first side of thesecond metal stack; and a conductive feature extending through thesecond substrate, the second metal stack, and to the third metal stacksuch that the conductive feature electrically couples the second metalstack to the third metal stack, and wherein the first side of the firstmetal stack includes a first metal portion and a first dielectricportion and the first side of the third metal stack includes a secondmetal portion and a second dielectric portion, and wherein the firstmetal portion directly interfaces with the second metal portion and thefirst dielectric portion directly interfaces with second dielectricportion.
 2. The device of claim 1, wherein the conductive featureextends through the second photodiode region.
 3. The device of claim 1,wherein the second side of the third substrate includes a first materiallayer and the first side of the second metal stack includes a secondmaterial layer, wherein the first material layer directly interfaceswith the second material layer, and wherein the first and secondmaterial layers are formed of the same material.
 4. The device of claim3, wherein the first and second material layers are formed of adielectric material.
 5. The device of claim 1, further comprisinganother conductive feature extending through the second substrate, thesecond metal stack, and to the third metal stack such that theconductive feature electrically couples the second metal stack to thethird metal stack.
 6. The device of claim 5, wherein a portion of thesecond photodiode region extends from the conductive feature to theanother conductive feature.
 7. A device comprising: a firstbackside-illuminate (BSI) image sensor element having a first substrateand a first metal stack disposed over a first side of the firstsubstrate, the first substrate including a first photodiode region; asecond BSI image sensor element having a second substrate and a secondmetal stack disposed over a first side of the second substrate, thesecond substrate including a second photodiode region; a third elementdisposed between the first and second BSI image sensors and having athird substrate and a third metal stack disposed over a first side ofthe third substrate, wherein a first side of the first metal stack isbonded to a first side of the third metal stack and a second side of thethird substrate is bonded to a first side of the second metal stack; anda first conductive feature extending through the first photodioderegion, the first metal stack, and to the third metal stack; and asecond conductive feature extending through the second photodioderegion, the second metal stack, and to the third metal stack.
 8. Thedevice of claim 7, wherein the third metal stack includes a conductivepad having a top surface and opposing bottom surface, and wherein thefirst conductive feature physically contacts the top surface of theconductive pad and the second conductive feature physically contacts thebottom surface of the conductive pad.
 9. The device of claim 7, whereinthe second side of the third substrate includes a first material layerand the first side of the second metal stack includes a second materiallayer, wherein the first material layer directly interfaces with thesecond material layer, and wherein the first and second material layersare formed of the same material.
 10. The device of claim 9, wherein thefirst and second material layers are formed of a first dielectricmaterial.
 11. The device of claim 10, wherein the first side of thefirst metal stack includes a third material layer and the first side ofthe third metal stack includes a fourth material layer, wherein thethird material layer directly interfaces with the fourth material layer,and wherein the third and fourth material layers are formed of the samematerial.
 12. The device of claim 11, wherein the third and fourthmaterial layers are formed of a second dielectric material.
 13. Thedevice of claim 12, wherein the second dielectric material is selectedfrom the group consisting of a silicon oxide material and a siliconoxynitride material.
 14. A device comprising: a firstbackside-illuminate (BSI) image sensor, wherein the first BSI imagesensor includes a first substrate and a first metal stack disposed overa first side of the first substrate, the first substrate includes aphotodiode region between the first side and a second side of the firstsubstrate, the first metal stack is operatively coupled to the firstsubstrate, and the first metal stack includes a first material layer ata first side of the first metal stack; a second BSI image sensor,wherein the second BSI image sensor includes a second substrate and asecond metal stack disposed over a first side of the second substrate,the second substrate includes a photodiode region between the first sideand a second side of the second substrate, the second metal stack isoperatively coupled to the second substrate, and the second metal stackincludes a second material layer at a first side of the second metalstack; and a third element, wherein the third element includes a thirdsubstrate and a third metal stack disposed over a first side of thethird substrate, the third substrate includes an active region, and thethird metal stack includes a third material layer at a first side of thethird metal stack, and wherein the first side of the first metal stackis bonded to the first side of the third metal stack such that the firstmetal stack is electrically coupled to the third metal stack, andwherein the first side of the second metal stack is disposed over asecond side of the third substrate such that the second metal stack iselectrically coupled to the third metal stack.
 15. The device of claim14, further comprising a passivation layer disposed between the firstside of the second metal stack and the second side of the thirdsubstrate, and a conductive feature extending from the first side of thesecond metal stack to the second side of the third substrate.
 16. Thedevice of claim 14, wherein the first metal stack further includes afourth material layer that is different from the first material layer,and wherein the third metal stack includes a fifth material layer thatis different from the third material layer, and wherein the first metalstack is bonded to the third metal stack such that the first materiallayer directly interfaces with the third material layer and the fourthmaterial layer directly interfaced with the fifth material layer. 17.The device of claim 14, wherein the first metal stack includes a firstplurality of conductive pads in the first material layer, wherein thethird metal stack includes a third plurality of conductive pads in thethird material layer, and wherein the first plurality of conductive padsis electrically coupled to the third plurality of conductive pads. 18.The device of claim 17, wherein the first and the third pluralities ofconductive pads are formed of copper, wherein the first material layerincludes one of silicon, silicon oxide, and silicon oxynitride, andwherein the third material layer includes one of silicon, silicon oxide,and silicon oxynitride.
 19. The device of claim 14, wherein the first,second, and third substrate are silicon substrates.
 20. The device ofclaim 1, wherein at least one of the first image sensor and the secondimage sensor is a backside-illuminated image sensor.